Field of the Invention
The instant invention relates to digital line receiver circuits and more particularly circuits capable of representing polarity changes of a digital signal which has been transmitted over a long cable and is thereby substantially distorted by the attenuation of the high frequency components of the pulses by the skin effect phenomenon in the center conductor of the transmitting cable. The invention is particularly useful for the reception of composite digital video signals since such signals have a wide range of pulse width. The varying pulse width prevents the use of a single threshold level which can allow detection of the short as well as the long pulses. Even is such detection were feasible, it would cause severe distortion of the time duration of the detected pulses. Such time duration distortion would be very noticeable on a television screen.
It has been found that, although there is severe amplitude attenuation of the short pulses by the transmitting cable, one property of the pulses is faithfully transmitted by the cable. This property is the relative time at which the input signal to the cable changed state (i.e., from a logical one to a logical zero or vice versa). At the output end of the cable, a change of state of the input signal produces a change in the sign of the slope of the output signal (i.e., the slope changes from a positive value to a slope with a negative value or vice versa).